This invention relates to a method for correction for chromatic aberration and an optical exposure apparatus used in fabrication of semiconductor devices or the like by using the correction method, and more particularly to a reduction-projection excimer exposure apparatus devised for achievement of ultrafine processing by photolithographic technology in fabrication of semiconductor devices.
Presently, there is already commercial reduction-projection exposure apparatus (stepper) using a superhigh pressure mercury lamp as a light source for ultrafine processing of semiconductor devices, in particular, LSI and VLSI. In conventional steppers, however, since a g-line (436 nm) or i-line (365 nm) superhigh pressure mercury lamp is used, resolution has been limited to 1.2 .mu.m in the g-line or about 0.8 .mu.m in the i-line. At such wavelengths, it is almost impossible to obtain a resolution of 0.5 .mu.m which is said to be required in the future for fabrication of 4 Mbit devices DRAM (dynamic random access memory) or 16 Mbit devices DRAM.
Recently, therefore, it has been considered to develop an exposure apparatus using an excimer light source such as XeCl (308 nm), KrF (249 nm) and ArF (193 nm) having a shorter wavelength as compared with g-line mercury lamps or i-line.
However, reduction-projection exposure using an excimer laser involves the following two problems.
(1) A Problem as to the half-width value of the wavelength distribution of light emitted by the excimer laser and achromaticity of the reduction-projection lens (relation between speckle and resolution).
(2) A Problem of selection of alignment wavelength.
The former problem may be solved by providing an optical system for adjusting vertical mode while distributing the lateral mode of excimer laser light. However, in the latter problem of selection of alignment light, it is preferable to employ a method of aligning directly marks on a reticle (mask) and wafer passing a wafer through a reduction-projection lens by using light at the same wavelength as the exposure wavelength in order to enhance alignment precision (through-the-lens method, or TTL method), which is advantageous in that lens design is facilitated. Therefore, since exposure wavelength is in the ultraviolet region, alignment cannot be achieved in pattern recognition of visible light such as e-line and d-line using a conventional video camera. Yet, when aligned with a line having the same wavelength as the exposure wavelength, the resist of the alignment key part is exposed to the alignment light, and, as a result, the alignment key on the wafer is broken by one operation. Still more, in order to solve this disadvantage, if the wavelength of the alignment light deviates from the exposure wavelength, the focal position at the exposure wavelength and that of the alignment wavelength are caused to deviate due to chromatic aberration, which actually results in alignment failure.